Editing Climate variability and change (section)

== Climate history ==
{{See also|List of periods and events in climate history|Paleoclimatology}}
Various [[climate forcing]]s are typically in flux throughout [[geologic time]], and some processes of the Earth's temperature may be [[homeostasis|self-regulating]]. For example, during the [[Snowball Earth]] period, large glacial ice sheets spanned to Earth's equator, covering nearly its entire surface, and very high [[albedo]] created extremely low temperatures, while the accumulation of snow and ice likely removed carbon dioxide through [[Deposition (chemistry)|atmospheric deposition]]. However, the absence of [[plant cover]] to absorb atmospheric CO<sub>2</sub> emitted by volcanoes meant that the greenhouse gas could accumulate in the atmosphere. There was also an absence of exposed silicate rocks, which use CO<sub>2</sub> when they undergo weathering. This created a warming that later melted the ice and brought Earth's temperature back up.

=== Paleo-eocene thermal maximum ===
[[File:65 Myr Climate Change.png|thumb|upright=1.35|right|Climate changes over the past 65 million years, using proxy data including [[Oxygen-18]] ratios from [[foraminifera]].]]
The [[Paleocene–Eocene Thermal Maximum]] (PETM) was a time period with more than 5–8&nbsp;°C global average temperature rise across the event.<ref name="McInherney-2011">{{cite journal|author=McInherney, F.A..|author2=Wing, S.|year=2011|title=A perturbation of carbon cycle, climate, and biosphere with implications for the future|url=http://www.whoi.edu/fileserver.do?id=136084&pt=2&p=148709|journal=Annual Review of Earth and Planetary Sciences|volume=39|issue=1 |pages=489–516|bibcode=2011AREPS..39..489M|doi=10.1146/annurev-earth-040610-133431|access-date=26 October 2019|archive-date=14 September 2016|archive-url=https://web.archive.org/web/20160914003526/http://www.whoi.edu/fileserver.do?id=136084&pt=2&p=148709|url-status=live|url-access=subscription}}</ref> This climate event occurred at the time boundary of the [[Paleocene]] and [[Eocene]] geological [[Epoch (geology)|epochs]].<ref name="Evans-2008">{{cite journal|author=Westerhold, T..|author2=Röhl, U.|author3=Raffi, I.|author4=Fornaciari, E.|author5=Monechi, S.|author6=Reale, V.|author7=Bowles, J.|author8=Evans, H. F.|year=2008|title=Astronomical calibration of the Paleocene time|url=https://www.geo.arizona.edu/~reiners/fortransfer6/WesterholdEtAl_PPP2008.pdf |archive-url=https://web.archive.org/web/20170809094938/http://www.geo.arizona.edu/~reiners/fortransfer6/WesterholdEtAl_PPP2008.pdf |archive-date=2017-08-09 |url-status=live|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=257|issue=4|pages=377–403|bibcode=2008PPP...257..377W|doi=10.1016/j.palaeo.2007.09.016}}</ref> During the event large amounts of [[methane]] was released, a potent greenhouse gas.{{Sfn|Burroughs|2007|p=|pp=190–91}} The PETM represents a "case study" for modern climate change as in the greenhouse gases were released in a geologically relatively short amount of time.<ref name="McInherney-2011"/> During the PETM, a mass extinction of organisms in the deep ocean took place.<ref>{{Cite journal|last1=Ivany|first1=Linda C.|last2=Pietsch|first2=Carlie|last3=Handley|first3=John C.|last4=Lockwood|first4=Rowan|last5=Allmon|first5=Warren D.|last6=Sessa|first6=Jocelyn A.|date=1 September 2018|title=Little lasting impact of the Paleocene-Eocene Thermal Maximum on shallow marine molluscan faunas|journal=Science Advances|language=en|volume=4|issue=9|pages=eaat5528|doi=10.1126/sciadv.aat5528|issn=2375-2548|pmid=30191179|pmc=6124918|bibcode=2018SciA....4.5528I}}</ref>

=== The Cenozoic ===
Throughout the [[Cenozoic]], multiple climate forcings led to warming and cooling of the atmosphere, which led to the early formation of the [[Antarctic ice sheet]], subsequent melting, and its later reglaciation. The temperature changes occurred somewhat suddenly, at carbon dioxide concentrations of about 600–760 ppm and temperatures approximately 4&nbsp;°C warmer than today. During the Pleistocene, cycles of glaciations and interglacials occurred on cycles of roughly 100,000&nbsp;years, but may stay longer within an interglacial when [[orbital eccentricity]] approaches zero, as during the current interglacial. Previous interglacials such as the [[Eemian interglacial|Eemian]] phase created temperatures higher than today, higher sea levels, and some partial melting of the [[West Antarctic ice sheet]].

Climatological temperatures substantially affect cloud cover and precipitation. At lower temperatures, air can hold less water vapour, which can lead to decreased precipitation.<ref>{{Cite journal|last1=Haerter|first1=Jan O.|last2=Moseley|first2=Christopher|last3=Berg|first3=Peter|date=2013|title=Strong increase in convective precipitation in response to higher temperatures|journal=Nature Geoscience|volume=6|issue=3|pages=181–85|doi=10.1038/ngeo1731|bibcode=2013NatGe...6..181B|issn=1752-0908}}</ref> During the [[Last Glacial Maximum]] of 18,000 years ago, thermal-driven [[evaporation]] from the oceans onto continental landmasses was low, causing large areas of extreme desert, including [[polar desert]]s (cold but with low rates of cloud cover and precipitation).<ref name="OakRidge-1997" /> In contrast, the world's climate was cloudier and wetter than today near the start of the warm [[Atlantic period|Atlantic Period]] of 8000 years ago.<ref name="OakRidge-1997" />

==== The Holocene ====
[[File:Holocene Temperature Variations.png|right|thumb|upright=1.35|Temperature change over the past 12 000 years, from various sources. The thick black curve is an average.]]
The [[Holocene]] is characterized by a long-term cooling starting after the [[Holocene climatic optimum|Holocene Optimum]], when temperatures were probably only just below current temperatures (second decade of the 21st century),<ref>{{Cite journal|last1=Kaufman|first1=Darrell|last2=McKay|first2=Nicholas|last3=Routson|first3=Cody|last4=Erb|first4=Michael|last5=Dätwyler|first5=Christoph|last6=Sommer|first6=Philipp S.|last7=Heiri|first7=Oliver|last8=Davis|first8=Basil|date=30 June 2020|title=Holocene global mean surface temperature, a multi-method reconstruction approach|journal=Scientific Data|language=en|volume=7|issue=1|page=201|doi=10.1038/s41597-020-0530-7|pmid=32606396|pmc=7327079|bibcode=2020NatSD...7..201K|issn=2052-4463|doi-access=free}}</ref> and a strong [[African Monsoon]] created grassland conditions in the [[Sahara]] during the [[Neolithic Subpluvial]]. Since that time, several [[stadial|cooling events]] have occurred, including:

*the [[Piora Oscillation]]
*the [[Middle Bronze Age Cold Epoch]]
*the [[Iron Age Cold Epoch]]
*the [[Little Ice Age]]
*the phase of cooling c. 1940–1970, which led to [[global cooling]] hypothesis

In contrast, several warm periods have also taken place, and they include but are not limited to:
*a warm period during the apex of the [[Minoan civilization]]
*the [[Roman Warm Period]]
*the [[Medieval Warm Period]]
*[[Modern Warming|Modern warming]] during the 20th century

Certain effects have occurred during these cycles. For example, during the Medieval Warm Period, the [[American Midwest]] was in drought, including the [[Sand Hills (Nebraska)|Sand Hills of Nebraska]] which were active [[sand dune]]s. The [[black death]] plague of ''[[Yersinia pestis]]'' also occurred during Medieval temperature fluctuations, and may be related to changing climates.

Solar activity may have contributed to part of the modern warming that peaked in the 1930s. However, solar cycles fail to account for warming observed since the 1980s to the present day.{{Citation needed|date=September 2016}} Events such as the opening of the [[Northwest Passage]] and recent record low ice minima of the modern [[Arctic shrinkage]] have not taken place for at least several centuries, as early explorers were all unable to make an Arctic crossing, even in summer. Shifts in [[biome]]s and habitat ranges are also unprecedented, occurring at rates that do not coincide with known climate oscillations {{Citation needed|date=September 2016}}.

=== Modern climate change and global warming ===
{{main|Climate change}}

As a consequence of humans emitting [[greenhouse gas]]es, [[Surface air temperature|global surface temperatures]] have started rising. Global warming is an aspect of modern climate change, a term that also includes the observed changes in precipitation, storm tracks and cloudiness. As a consequence, glaciers worldwide have been found to be [[The Retreat of Glaciers Since 1850|shrinking significantly]].<ref name="Zemp-2008">{{cite report|url=http://www.grid.unep.ch/glaciers/pdfs/summary.pdf|title=United Nations Environment Programme&nbsp;– Global Glacier Changes: facts and figures|last=Zemp|first=M.|author2=I.Roer|author3=A.Kääb|author4=M.Hoelzle|author5=F.Paul|author6=W. Haeberli|access-date=21 June 2009|archive-url=https://web.archive.org/web/20090325100332/http://www.grid.unep.ch/glaciers/pdfs/summary.pdf|year=2008|archive-date=25 March 2009|url-status=dead}}</ref><ref name="EPA-2016">{{cite web|url=https://www.epa.gov/climate-indicators/climate-change-indicators-glaciers|title=Climate Change Indicators: Glaciers|last=EPA, OA|first=US|website=US EPA|date=July 2016|access-date=26 January 2018|archive-date=29 September 2019|archive-url=https://web.archive.org/web/20190929003522/https://www.epa.gov/climate-indicators/climate-change-indicators-glaciers|url-status=live}}</ref> Land ice sheets in both [[Antarctica]] and [[Greenland]] have been losing mass since 2002 and have seen an acceleration of ice mass loss since 2009.<ref>{{cite web|url=https://climate.nasa.gov/vital-signs/land-ice/|title=Land ice – NASA Global Climate Change|access-date=10 December 2017|archive-date=23 February 2017|archive-url=https://web.archive.org/web/20170223211832/https://climate.nasa.gov/vital-signs/land-ice/|url-status=live}}</ref> Global sea levels have been rising as a consequence of thermal expansion and ice melt. The decline in Arctic sea ice, both in extent and thickness, over the last several decades is further evidence for rapid climate change.<ref>{{cite web|url=https://climate.nasa.gov/evidence/|title=Climate Change: How do we know?|editor1-last=Shaftel|editor1-first=Holly|website=NASA Global Climate Change|publisher=Earth Science Communications Team at NASA's Jet Propulsion Laboratory|access-date=16 December 2017|archive-date=18 December 2019|archive-url=https://web.archive.org/web/20191218104252/https://climate.nasa.gov/evidence/|url-status=live}}</ref>

==== Variability between regions {{anchor|Contemporaneous regional variability}} ====
{{Gallery
|align=right | height=150 |mode=packed
|title= Examples of regional climate variability

| File:Land vs Ocean Temperature.svg
 | '''Land-ocean.''' Surface air temperatures over land masses have been increasing faster than those over the ocean,<ref name="NASA GISS">{{cite web |title=GISS Surface Temperature Analysis (v4) / Annual Mean Temperature Change over Land and over Ocean |url=https://data.giss.nasa.gov/gistemp/graphs_v4/ |website=NASA GISS |archive-url=https://web.archive.org/web/20200416074510/https://data.giss.nasa.gov/gistemp/graphs_v4/ |archive-date=16 April 2020 |url-status=live}}</ref> the ocean absorbing about 90% of excess heat.<ref name="Harvey-2018">{{cite magazine |last1=Harvey |first1=Chelsea |title=The Oceans Are Heating Up Faster Than Expected |url=https://www.scientificamerican.com/article/the-oceans-are-heating-up-faster-than-expected/ |magazine=Scientific American |date=1 November 2018 |archive-url=https://web.archive.org/web/20200303222236/https://www.scientificamerican.com/article/the-oceans-are-heating-up-faster-than-expected/ |archive-date=3 March 2020 |url-status=live }} Data from [https://web.archive.org/web/20200416074510/https://data.giss.nasa.gov/gistemp/graphs_v4/ NASA GISS].</ref>

|File:20200505 Global warming variability - Northern vs Southern hemispheres.svg
 | '''Hemispheres.''' The Hemispheres' average temperature changes<ref name="NASA GISS-3">{{cite web |title=GISS Surface Temperature Analysis (v4) / Annual Mean Temperature Change for Hemispheres |url=https://data.giss.nasa.gov/gistemp/graphs_v4/ |website=NASA GISS |archive-url=https://web.archive.org/web/20200416074510/https://data.giss.nasa.gov/gistemp/graphs_v4/ |archive-date=16 April 2020 |url-status=live}}</ref>  have diverged because of the North's greater percentage of landmass, and due to global ocean currents.<ref name="Freedman-2013">{{cite web |last1=Freedman |first1=Andrew |title=In Warming, Northern Hemisphere is Outpacing the South |url=https://www.climatecentral.org/news/in-global-warming-northern-hemisphere-is-outpacing-the-south-15850 |website=Climate Central |archive-url=https://web.archive.org/web/20191031123759/https://www.climatecentral.org/news/in-global-warming-northern-hemisphere-is-outpacing-the-south-15850 |archive-date=31 October 2019 |date=9 April 2013 |url-status=live }}</ref>

| File:20200314 Temperature changes for three latitude bands (5MA, 1880- ) GISS.svg
 | '''Latitude bands.''' Three latitude bands that respectively cover 30, 40 and 30 percent of the global surface area show mutually distinct temperature growth patterns in recent decades.<ref name="NASA GISS-2">{{cite web |title=GISS Surface Temperature Analysis (v4) / Temperature Change for Three Latitude Bands |url=https://data.giss.nasa.gov/gistemp/graphs_v4/ |website=NASA GISS |archive-url=https://web.archive.org/web/20200416074510/https://data.giss.nasa.gov/gistemp/graphs_v4/ |archive-date=16 April 2020 |url-status=live}}</ref>

| File:1960- Warming stripes global temperature graphic - atmospheric heights and ocean depths.png
 | '''Altitude.''' A [[warming stripes]] graphic ({{blue|blues}} denote cool, {{red|reds}} denote warm) shows how the greenhouse effect traps heat in the lower atmosphere and oceans, so that the upper atmosphere, receiving less reflected energy, cools.<ref name=AMS_20250501>{{cite journal |last1=Hawkins |first1=Ed |last2=Williams |first2=Richard G. |last3=Young |first3=Paul J. |last4=Berardelli |first4=Jeff |last5=Burgess |first5=Samantha N. |last6=Highwood |first6=Ellie |last7=Randel |first7=William |last8=Roussenov |first8=Vassil |last9=Smith |first9=Doug |last10=Placky |first10=Bernadette Woods |title=Warming Stripes Spark Climate Conversations: From the Ocean to the Stratosphere |journal=Bulletin of the American Meteorological Society |volume=6 |issue=5 |date=1 May 2025 |pages=E964-E970 |doi=10.1175/BAMS-D-24-0212.1}}</ref><ref name=Hawkins-2019>{{cite web |last1=Hawkins |first1=Ed |title=Atmospheric temperature trends |url=http://www.climate-lab-book.ac.uk/2019/atmospheric-temperature-trends/ |website=Climate Lab Book |archive-url=https://web.archive.org/web/20190912192530/http://www.climate-lab-book.ac.uk/2019/atmospheric-temperature-trends/ |archive-date=12 September 2019 |date=12 September 2019 |url-status=live }} (Higher-altitude cooling differences attributed to ozone depletion and greenhouse gas increases; spikes occurred with volcanic eruptions of 1982–83 (El Chichón) and 1991–92 (Pinatubo).)</ref>
 
| File:20200505 Global warming variability - global vs Caribbean.svg 
 | '''Global versus regional.''' For geographical and statistical reasons, larger year-to-year variations are expected<ref name="Meduna-2018">{{cite news |last1=Meduna |first1=Veronika |title=The climate visualisations that leave no room for doubt or denial |url=https://thespinoff.co.nz/science/17-09-2018/the-climate-visualisations-that-leave-no-room-for-doubt-or-denial/ |work=The Spinoff |date=17 September 2018 |archive-url=https://web.archive.org/web/20190517104250/https://thespinoff.co.nz/science/17-09-2018/the-climate-visualisations-that-leave-no-room-for-doubt-or-denial/ |archive-date=17 May 2019 |location=New Zealand |url-status=live }}</ref> for localized geographic regions (e.g., the Caribbean) than for global averages.<ref name="NCDC_NOAA">{{cite web |title=Climate at a Glance / Global Time Series |url=https://www.ncdc.noaa.gov/cag/global/time-series/globe/land_ocean/12/12/1880-2019 |website=NCDC / NOAA |archive-url=https://web.archive.org/web/20200223062050/https://www.ncdc.noaa.gov/cag/global/time-series/globe/land_ocean/12/12/1880-2019 |archive-date=23 February 2020 |url-status=live}}</ref>

| File:20200509 Emergence of temperatures from range of normal historical variability - tropical vs northern Americas (Hawkins).gif 
 | '''Relative deviation.''' Though northern America has warmed more than its tropics, the tropics have more clearly departed from normal historical variability (colored bands: 1σ, 2σ standard deviations).<ref name="Hawkins-2020">{{cite web |last1=Hawkins |first1=Ed |title=From the familiar to the unknown |url=https://www.climate-lab-book.ac.uk/2020/from-the-familiar-to-the-unknown/ |website=Climate Lab Book (professional blog) |archive-url=https://web.archive.org/web/20200423232229/https://www.climate-lab-book.ac.uk/2020/from-the-familiar-to-the-unknown/ |archive-date=23 April 2020 |date=10 March 2020 |url-status=live }} ([https://web.archive.org/web/20200502073245/http://www.climate-lab-book.ac.uk/files/2020/03/emerge_example.png Direct link to image]; Hawkins credits [[Berkeley Earth]] for data.) "The emergence of observed temperature changes over both land and ocean is clearest in tropical regions, in contrast to the regions of largest change which are in the northern extra-tropics. As an illustration, northern America has warmed more than tropical America, but the changes in the tropics are more apparent and have more clearly emerged from the range of historical variability. The year-to-year variations in the higher latitudes have made it harder to distinguish the long-term changes."</ref>
}}
[[File:1880- Global warming by latitude zone - NASA - GISS data.webm|thumb| upright=1.15| Global warming has varied substantially by latitude, with the northernmost latitude zones experiencing the largest temperature increases.]]
In addition to global climate variability and global climate change over time, numerous climatic variations occur contemporaneously across different physical regions.

The oceans' absorption of about 90% of excess heat has helped to cause land surface temperatures to grow more rapidly than sea surface temperatures.<ref name="Harvey-2018"/> The Northern Hemisphere, having a larger landmass-to-ocean ratio than the Southern Hemisphere, shows greater average temperature increases.<ref name="Freedman-2013"/> Variations across different latitude bands also reflect this divergence in average temperature increase, with the temperature increase of northern [[wikt:extratropics|extratropics]] exceeding that of the tropics, which in turn exceeds that of the southern extratropics.<ref name="NASA GISS-2"/>

Upper regions of the atmosphere have been cooling contemporaneously with a warming in the lower atmosphere, confirming the action of the greenhouse effect and ozone depletion.<ref name="Hawkins-2019"/>

Observed regional climatic variations confirm predictions concerning ongoing changes, for example, by contrasting (smoother) year-to-year global variations with (more volatile) year-to-year variations in localized regions.<ref name="Meduna-2018"/> Conversely, comparing different regions' warming patterns to their respective historical variabilities, allows the raw magnitudes of temperature changes to be placed in the perspective of what is normal variability for each region.<ref name="Hawkins-2020"/>

Regional variability observations permit study of regionalized [[Tipping points in the climate system|climate tipping point]]s such as rainforest loss, ice sheet and sea ice melt, and permafrost thawing.<ref name="Lenton-2019">{{Cite journal|last1=Lenton|first1=Timothy M.|last2=Rockström|first2=Johan |last3=Gaffney|first3=Owen|last4=Rahmstorf|first4=Stefan|last5=Richardson|first5=Katherine|last6=Steffen |first6=Will|last7=Schellnhuber|first7=Hans Joachim|date=27 November 2019|title=Climate tipping points – too risky to bet against|journal=Nature|language=en|volume=575|issue=7784|pages=592–595|pmid=31776487 |bibcode=2019Natur.575..592L|doi=10.1038/d41586-019-03595-0|doi-access=free|hdl=10871/40141|hdl-access=free}} Correction dated 9 April 2020</ref> Such distinctions underlie research into a possible [[abrupt climate change|global cascade of tipping points]].<ref name="Lenton-2019" />